Right ventricular dilatation due to an increase in right ventricular volume, but without an increase in pulmonary artery or right ventricular pressure, as occurs in patients with atrial septal defects, is associated with electrocardiographic changes that are much different than those with accompany an increase in right ventricular systolic pressure. In large part, the ECG of the volume-overloaded right ventricle reflects the stretching of the right bundle branch and the increased time required for right ventricular depolarization. Its predominant feature is right bundle branch block.